There are lots of punch recipes that call for sherbet, maybe you could find blue sherbet or even sub in blue moon ice cream in one of those recipes?

Or try to find a neutral colored punch and mix in either a blue juice drink (like Hawaiian punch in a different flavor) or even add a blue/ice blue lemonade pack of koolaid. Usually the alcoholic drinks get their bright blue color from the liquor that is added, like blue Curacao, so there probably aren’t a ton of recipes for nonalcoholic blue punch. I think if you’re willing to do a couple trials, then you could alter any basic recipe to fit your color scheme.

I’m not sure if pureed blueberries might also be an option to mix in, but it might wind up too purple from them. Blue food coloring might be a good option – it wouldn’t change the flavor, but be careful not to use too much (everyone having blue stained teeth would not be very attractive in your pictures.)

check out Monin brand mixers to see if they have anything blue. if all else fails, just put blue dye into whatever it is that you want to serve so that it is aesthetically pleasing. I saw blue lemonade before!